Automatic pitch control for horizontal axis wind turbines

ABSTRACT

A self setting and self powered system for adjusting the blade angles of a wind turbine such that they have a high angle of attack when parked to promote early start up, move to their ideal setting angle for normal running, can respond to gusts and lulls with appropriate changes, and feathers the blades to limit the rpm and reduce loads in storm conditions. The system balances the thrust loads on the turbine against centripetal forces generated by masses reacting to the rpm, increasing the blade pitch with gusts to increase torque and hence rotational acceleration and reducing blade pitch in lulls to conserve rotational momentum, and uses springs to establish the starting position and the rpm limit appropriate for the generator mechanism.

The present application pertains to a novel means by which the bladepitch angle of a wind turbine can be automatically set for optimum poweroutput in changing wind conditions.

FIELD OF THE INVENTION

The present invention relates to a mechanism that can be employed byHorizontal Axis Wind Turbines (HAWT) to set the optimum blade pitchangle for the differing conditions they are likely to encounter in orderto optimize their power yield.

Whereas large turbines can justify complex servo driven pitch adjustmentsystems supplied with wind condition data from sensors, these are costlyand power hungry. Small scale turbines consequently primarily use simplefixed pitch systems.

There have been proposed a number of compromise solutions that are selfpowered from the centripetal force on the turbine blades, but these areoften poor compromises, not adequately optimizing the start up conditionand the overload condition. Also none of them can react quickly to bestutilize the transient changes in wind speed such as gusts and lulls.

BACKGROUND OF THE INVENTION

All large scale turbines currently use a variable pitch adjustmentsystem to set the best blade angle of attack to match the prevailingwind conditions. This ideal setting angle for optimum lift/dragperformance of the selected aerofoil is added to the angle defined bythe Tip Speed Ratio (TSR) which is the blade tip speed divided by thewind speed. Studies published by Mutschler and Hoffman titled“Comparison of Wind Turbines Regarding their Energy Generation” indicatethat variable pitch can deliver around 20% more power than fixed pitchsystems and up to 38% for small high TSR systems. Gains are mostpronounced where the average wind speed is quite low and turbulencehigh—as with smaller turbines on lower towers.

It can be shown that a small turbine operating at an average wind speedof 6 m/s would need to adjust its angle of attack by around 7 degreesfaced with a gust or lull of 3 m/s to stay at its optimum performance.Failure to do so in a lull would reduce its coefficient of lift from ˜2to ˜1.3 and in a gust its drag coefficient would increase from ˜0.03 to˜0.075 with only a small increase in lift. If the fluctuation was 6 m/s,in a lull the turbine would act as a fan and waste its angular momentumaccelerating the air and in a gust the list coefficient would drop to˜1.8 with drag climbing hugely to ˜0.15

Large systems use transducer data on wind speed and turbine rpm toinstruct electrically powered servos to adjust the blade pitchproportionately. It is worth noting though that the high blade inertiaand the relatively slow speed of servos does not enable them to optimizeon as quick a wind speed variation as may be experienced. Mutschler usesa simulation with an average 20% turbulence deviation and changes thatoccur between their limits in ˜10 seconds. Actual wind data suggeststhese changes are much faster as FIGS. 1 and 2 that show a typicalpattern over a 10 second period in both normal and turbulent conditions.With energy being proportional to the cube of the wind speed, thefluctuations equate to a doubling or tripling of energy within a fewseconds. They indicate that if pitch change could be effected quicklyenough further power gains are possible.

The compromise self powered solution that uses centripetal force toadjust the pitch angle cannot react quickly as the rotors need time toaccelerate. Its effect is to reduce the pitch and hence increase the TSRin high winds. This does not assist significantly with start up and canonly react to overload by putting the blades into a stall which whilereducing rpm does so at the cost of high axial thrust loads and so putsextra strain on the tower.

Various other means have been proposed such as allowing the blades topivot at a skew angle such that when gusts blow them back they increasethe blades' angle of attack. This response is difficult to gear tooptimum setting levels and while helping marginally with start up doesnot provide an overload solution.

Most small systems turn away from excess wind by using a rudder to skewthemselves sideways where they become deliberately inefficient andturbulent, or have other means to spill the excess power. Although thegenerator may be protected from spinning too fast, the windmillstructure still has to bare the increased wind load. The result ofoverload protection is often a reduction in power output just when theenergy density is the greatest.

OBJECTS OF THE INVENTION

A principal object of this invention, therefore, is to provide a meansto automatically set the blade pitch to a low TSR in order to delivermaximum torque at zero rotational speed for early turbine start up.

A further object of this invention is to increase the TSR as the turbineaccelerates and hold it stable at the ideal continuous runningcondition.

A further object of this invention is to quickly react to gusts ortransient increases in wind speed by reducing the TSR to increase thetorque and hence rate of rotational acceleration.

A further object of this invention is to quickly react to lulls ortransient decreases in wind speed by increasing the TSR to reduce thetorque and hence maintain rotational momentum.

A further object of this invention is to react to excessive wind speedin order not to exceed the safe rpm limit by reducing the TSR in theseconditions—akin to feathering the blades where they generate less drag.

A further object of this invention is to provide for all of the abovewith a self powered and relatively simple mechanism that can be built atlow cost.

Other and further objects will be explained hereinafter and moreparticularly delineated in the appended claims.

SUMMARY OF THE INVENTION

In summary, the invention proposes to utilize both the axial thrust onthe rotor and the centripetal forces related to its rotational velocityto achieve an optimum solution for all conditions.

The basic concept is that axial thrust pushes the turbine back along itsshaft causing the pitch to be increased and TSR lowered, while rpmrelated centripetal force pulls the turbine forward causing the TSR tobe increased.

The result of this in gusty conditions is that when the wind speedincreases faster than the turbine can accelerate, the force imbalancecauses the turbine to be displaced backwards and thereby reduces the TSRto avoid stalling and so to better utilize the available power asincreased torque.

Equally when there is a lull, the imbalance moves the turbine forwardincreasing the TSR and thereby reducing the torque and hence helping itconserve its rotational momentum.

To cause the turbine to move to a low TSR for early start up in lowwind, a spring is provided to push the turbine back, effectivelycooperating with any wind.

To cause the turbine to move towards a low TSR in order to limit the rpmin overload conditions and track the maximum safe power even as windcontinues to increase in speed, a further spring device is employed.This overload spring acts between the centripetal forcing element andthe turbine and thereby enables the turbine to move back in high windsirrespective of the amount of centripetal force being generated. It isset to a preload such that it begins to permit displacement after theknown thrust generated at the rpm limit is exceeded and at a spring rateappropriate to maintain that limit as the TSR is proportionatelyreduced.

Rather than utilize the blades to generate the balancing centripetalforce, in a preferred embodiment separate masses are used. The advantageof decoupling the centripetal force from the blades is that blade forcescould become excessively high and the increase in tip diameter as theymove out radially could be a problem where the blades run in a confinedduct as in a system where a diffuser is used to accelerate the windthrough the turbine.

Masses can be arranged to fly out axially, perhaps even surrounding theblade shaft, but a preferred embodiment is for them to swing up on armsmuch like what occurs with a speed governor.

This action can be arranged to cause a roller on the mass lever to actagainst a cam whose profile presents a local ramp angle which gears theamount of axial balancing force generated to the optimum for any givenrpm.

The scheme requires that the blade shaft is rotated in proportion to theaxial displacement of the hub. A preferred means of achieving this is byemploying two rollers attached to the fixed generator shaft acting ondouble sided cams attached to the blade shafts. The cam angleprogression can again be selected to vary the gearing so therelationship need not be linear. As the turbine is displaced, the camsare obliged to rotate as the rollers are in a fixed position withrespect to the generator shaft.

A further embodiment uses a belt firstly attached to the fixed part ofthe hub, then wrapped around the blade axles attached to the axiallydisplaceable part of the hub and then returned to the fixed part via aspring. As the blade set is displaced axially, the belts cause theblades to rotate on their shafts with the slack taken up by the springor belt lengthened by the spring.

In this embodiment the springs act as a preload device in not justmaintaining belt tension, but also in trying to bull the belt in theymove the blades set such that it increases their pitch angle and becomesmore feathered as would be appropriate in a start up condition. In doingso it also holds the centripetal masses in their closed position.

After start up, it is desirable for the TSR to rapidly increase to itsbest running mode, and yet the start up spring holds in place the massesthat generate the centripetal force necessary to cause this. Thecentripetal forces must therefore increase quicker than the thrustforces in order for them to become dominant and start to move the hubforwards. The displacement will continue as the turbine acceleratesuntil the centripetal forces are once again balanced by the growingthrust loads augmented by the spring.

The balance point is reached at the point where the turbine torque hasdecreased with increasing TSR such that it can no longer accelerate.This is helped by the start up spring which provides its axial force inproportion to the axial turbine displacement that sets the TSR. As such,the balance point reached tends to increase it's TSR a little as thewind speed increases (and the spring has a relatively smaller forcingeffect). This is helpful in providing more torque at lower speed tobetter match the characteristics of the attached generator.

To tailor this balance point to the ideal TSR the centripetal forces aregeared by a varying ramp angle on the CAM against which they act aspreviously explained.

It can now been seen that wind speed fluctuations will act to move thisforce balance point, and as quick as the wind thrust load changes.

With gusts the wind thrust displaces the turbine backwards and therebyprovides the power to turn the blades to a lower TSR enabling them toaccelerate faster instead of tending towards stall. With lulls thecentripetal masses using the stored rotational inertia of the turbinenow have less balancing wind thrust so pull the hub forwards and soreduce the blade pitch and hence the torque with a higher TSR instead oftending towards turning into a fan where rotational momentum is quicklylost.

The charm of these reactions is that they can occur quickly, beingpowered from large wind thrust and momentum power reserves.

Best mode and preferred designs and techniques will now be described.

DRAWINGS

The present invention can best be understood in conjunction with theaccompanying drawings, in which:

FIG. 1 shows a first data illustration;

FIG. 2 shows a second data illustration;

FIG. 3 shows a force balance relationship;

FIG. 4 shows a tip speed ratio relationship;

FIG. 5 Shows two isometric views of the mechanism. The left hand viewshows it in start up condition where the blades are set to providemaximum torque. The right hand view shows it in maximum rpm conditionwhere the weights have swung out, pulling the hub forward and settingthe blades suitable for a high TSR.

FIG. 6 Shows two top views with the upper blade system sectioned throughits driving CAM. The left hand view shows it in start up condition withthe blade CAM at its backwards limit and where it's offset between theCAM rollers has caused it to rotate to its clockwise limit. The righthand view shows the hub pulled forward, moving the CAM to itsanti-clockwise limit.

FIG. 7 Shows a side view section through the centre of the mechanism,shown in its start up condition.

FIG. 8 Shows a similar side view section, but with the mechanism at itsmaximum rpm condition.

FIG. 9 Shows a similar side view section, but in its overload conditionwhere the overload springs have compressed and allowed the hub to moveback despite the high centripetal force opposing it.

FIG. 10 Shows the embodiment where a belt is used to rotate the bladesaxles. In this view the belts have been pulled back by their springscausing the blades to move to a high pitch feathered position.

FIG. 11 Shows the same embodiment as FIG. 10 but now in a running modeat a higher rpm where the centripetal masses have swung out and pulledthe blade set forward, thereby stretching the springs that hold thebelts and consequently causing the belts to rotate the blade's axles.

In the drawings, preferred embodiments of the invention are illustratedby way of example, it being expressly understood that the descriptionand drawings are only for the purpose of illustration and preferreddesigns, and are not intended as a definition of the limits of theinvention.

PREFERRED EMBODIMENT OF THE INVENTION In FIG. 5

The mechanism is show with its frame (1) comprising two profile cutsheets with lengths of tubing welded in a radial pattern to support theblade axes. This embodiment supports five blades, but it can be seemthat other numbers are possible. The hub is pushed to the back of itsshaft by the spring (2), holding in the centripetal masses (3) thatswing on arms to drive the roller (4) against the CAM (5) toprogressively displace the hub forward as the weights swing out as shownby (12).

CAM roller supports (10) retain rollers (6) which act on the bladesetting CAMs (7). It can be seen that the blades (8) rotate on theiraxes into position (11) as the weights force the hub forwards.

In FIG. 6

The sectioned mechanism in the left hand view shows the back flange (14)that supports the CAM roller retainers (19) with their rollers (20) and(22) acting on the CAM (17) to set its degree of rotation about theblade axis. The roller (23) acts against the narrow wedge plate (24) andtransmits the drive torque from the frame (13) to the flange (14). Thisdevice enables it to be adjusted to a degree of bearing preload in therollers consistent with stiff control. The masses (15) are in their parkposition.

In the right hand view the masses (16) have swing out pulling the huband frame (25) forwards away from the flange (27) causing the CAM (18)to rotate the blades from their start up position (25) to their high rpmposition (26).

In FIG. 7

The sectioned view shows the flange (28) rigidly connected to thegenerator shaft and the shaft extension. Connected to the flange are theCAM roller retainers (32) holding the rollers e.g. (34) against the CAM(38).

The frame (30) includes a bore that permits it to slide along the shaftas required to achieve the force balance axial offset position. Theframe is held up tight against a further stepped tube (29) by high forcesprings (33) riding on a radial array of shafts with end stops (31)trapping one of the frame's profiled plates between it and the springs.As will be seen this constitutes the overload preload device.

The weight is show fully retracted by the action of the spring (37)displacing the CAM (36) and frame (29 & 30) as far back as possiblecausing the roller (35) to rise to its top position.

In FIG. 8

The sectioned view now shows the mechanism in its maximum forwardposition where the roller (46) in swinging upwards has pulled the CAM(48) forward, dragging the stepped tube (45) and frame (39) with it—asmay be found at the rpm limit. The start up preload spring (47) is nowfully compressed.

The blade rotation has been set for maximum rpm as the CAM (42) has beenrotated by the action of the rollers e.g. (40) retained by the part (43)against the flange that is part of the shaft extension (44).

The overload springs (38) once again trap the frame up tight against thestepped tube (45).

In FIG. 9

The same sectioned view is shown as in FIG. 4, but now the wind thrusthas overwhelmed the overload springs (38) such that the frame (39) canslide back with respect to the stepped tube (45) which retains thecentripetal CAM (48). In so doing they can change the blade angle suchthat the rpm limit is not exceeded. In hurricane force conditions thismay be as far back as the start up position where the blades will beessentially feathered for minimum drag.

In FIG. 10

The embodiment where belts e.g. (49) are used acting around the pulleyse.g. (51) tensioned by springs e.g. (50) such that in this case thesprings pull in the axially displaceable part of the hub that holds theblades (57) which in turn causes the centripetal masses (55) to be heldin their closed position.

In FIG. 11

The same embodiment as FIG. 6 is shown, but now the blade set (58) hasbeen forced forward by the centripetal masses (56) swinging out on theirlever arms as a result of the hub's rotation. The springs e.g. (52) havenow been stretched effectively lengthening the belts e.g. (53) whichrotate the pulleys e.g. (54) and consequently adjust the blade pitchangle in this case reducing it.

Further modifications of the invention will also occur to personsskilled in the art, and all such are deemed to fall within the spiritand scope of the invention as defined by the appended claims.

1. A self powered automatic pitch adjustment system for a turbine thatincludes a shaft and at least one blade characterized by a blade pitchangle, the system comprising: a mechanism for adjusting said blade pitchangle, said mechanism coupled with the shaft; an element for generatinga centripetal force when the turbine rotates and for resolving saidforce axially along the shaft, wherein an axial wind force on theturbine is balanced against the axially resolved centripetal force, saidelement coupled with said mechanism, such that when the wind forcebuilds faster than the centripetal force the blade pitch angle isincreased and when the wind force reduces faster than the centripetalforce the pitch blade angle is reduced.
 2. The system of claim 1,wherein the mechanism comprises a hub holding the at least one blade,wherein the hub is displaced alone the shaft in a rearward direction inresponse to an increasing wind force but is displaced along the shaft ina forward direction in response to an increasing centripetal force sothat it results in a displacement that determines the blade pitch angle.3. The system of claim 1, wherein the mechanism further comprises aspring force member adapted to act along the shaft to augment the windforce so as to return the at least one blades to a high blade pitchangle when both wind force and speed of rotation of the turbine are low.4. The system of claim 2, wherein the mechanism further comprises apreload device adapted to enable the hub to be displaceable along theshaft in said rearward direction, when a wind force is above a definedlevel, irrespective of the centripetal force.
 5. The system of claim 2,wherein: the element for generating a centripetal force comprises one ormore masses mounted on lever arms to swing radially outward, themechanism comprises means for transforming radial movement of the massesinto movement in said forward and rearward directions.
 6. The system ofclaim 1 wherein the centripetal force is generated by a plurality ofmasses constrained to move out radially from the shaft with a means totransform the radial displacement into an axial displacement along theshaft.
 7. The system of claim 2, wherein the mechanism furthercomprises: a cam coupled with the at least one blade; and one or morerollers fixed to the shaft and coupled with the cam; whereindisplacement of the hub causes said cam to be proportionately rotated bysaid one or more rollers.
 8. The system of claim 1, wherein thecentripetal forces are generated by allowing the blades to move outradially and where such motion is translated into an axial force tooppose the wind force.
 9. The system of claim 2, wherein the at leastone blade comprises an axle, and wherein the mechanism further comprisesa pulley on said axle and a belt attached to the hub and running aroundsaid pulley so that the net displacement of the hub causes the belt torotate the at least one blade about the axles proportionally to said netdisplacement.
 10. The system of claim 2, wherein the at least one bladecomprises an axle, and wherein the mechanism further comprises a crankattached to said axle and a push rods running from a fixed point on thehub to said cranks so that the net displacement of the hub causes thepush rods to rotate the blades about the axles proportionally to saidnet displacement.
 11. The system of claim 5, wherein the means fortransforming includes a belt or a chain attached to the hub.
 12. Thesystem of claim 5, wherein the means for transforming includes: a camattached to the hub, the cam having a profile; and a roller attached tothe element for generating a centripetal force and adapted to actagainst the profile when the element undergoes said radial movement. 13.The system of claim 5, wherein the means for transforming has at leastone characteristic that is selected to optimize a balance between theaxial wind force and the axially resolved centripetal force.
 14. Thesystem of claim 9, wherein at least one end of the belt is attached tothe hub via a spring force member.
 15. The system of claim 14, whereinthe spring force member is adapted to act along the shaft to augment thewind force so as to return the at least one blade to a high blade pitchangle when both wind force and speed of rotation of the turbine are low.16. A wind turbine comprising: a shaft; a hub rotatably mounted on theshaft and moveable along the shaft; at least two blades mounted on thehub, wherein a wind force on the blades causes the blades to rotateabout the shaft and to be forced in a first direction along the shaft; acentripetal element rotatable with the blades around the shaft,generating a centripetal force towards the shaft; a coupling mechanismfor balancing the centripetal force with a force on the hub in a seconddirection along the shaft opposite to the first direction; and amechanism for adjusting a pitch angle for each blade in response tomovement of the hub in the first and second directions.
 17. The turbineof claim 16, wherein the mechanism adjusts the pitch angle to arelatively steep pitch angle with movement in the first direction and arelatively shallow pitch angle with movement in the second direction.18. The turbine of claim 16, wherein the centripetal element comprises aplurality of centripetal masses and wherein the coupling mechanismcomprises lever arms that on which the centripetal masses areconstrained to move out radially from the shaft whereby the lever armstransform radial displacement into axial displacement.